This invention relates in general to heat exchangers and more particularly to a heat exchanger for transmitting thermal energy from one moving fluid to another having an improved core structure capable of accommodating maximum fluid flow with minimum pressure drop. p In general, heat exchangers wherein thermal energy is transferred from a higher temperature gaseous medium to a lower temperature gaseous medium are well known. More particularly, heat exchangers comprising a generally rectangular casing which houses a thermal transfer core, which core includes a sheet of heat conductive material folded upon itself dividing the interior of the casing into adjacent fluid flow passages, alternate ones of the passages defining a first conduit for conducting a first gaseous medium with the other passages defining a second conduit for conducting a second gaseous medium, are known. Such heat exchangers are generally illustrated in, for example, U.S. Pat. Nos. 2,576,213 to Chausson and 2,945,680 to Slemmons. These devices are useful as heat recovery or air conditioning apparatus for buildings, for passenger compartments of vehicles, or, in general, wherever the recovery of thermal energy otherwise wasted, would be beneficial.
The presently known heat exchangers of the type discussed hereinabove, however, are subject to several disadvantages due in the main to the particular structure of the heat exchanger core. Since the fluid flow passages are defined by the spaces between adjacent folds of a folded sheet of heat conductive material and since alternate, adjacent fluid flow passages serve to conduct first and second gaseous mediums which are generally at different pressures, a pressure differential exists over the relatively long flexible sheet which separates adjacent flow passages. This can cause flexion of the sheet towards the lower pressure area which will narrow or lessen the width of that passage thereby impeding the flow of the gaseous medium. Similarly, the high pressure fluid flow passages will be widened thereby upsetting the calculated flow through those passages. In order to overcome this disadvantage it has been proposed to provide spacers between adjacent folds of the heat conductive sheet. These spacers are employed to maintain the spacing between the adjacent folds. For example, it has been suggested to form dimples in the alternate folds of the heat transfer sheet having a height equal to the desired clearance between adjacent folds which define the low pressure fluid flow passages to prevent this collapse due to the pressure differential. Further, it has been suggested to provide more widely spaced projections having a height equal to the desired clearance between adjacent folds defining the high pressure fluid flow passages to maintain the desired clearance in these passages.
This proposed structure has not, however, proved entirely satisfactory. It has been found that to effectively prevent collapse of the low pressure fluid passages according to the previously proposed techniques, a large number of dimples extending into these passages were necessary. These dimples, however, obstructed the efficient flow of the gaseous medium through the fluid flow passages giving rise to losses which rendered the system too inefficient for practical use. Further, where large projections were employed in the high pressure fluid flow passages, they also were found to intermittently block the flow of the fluid forcing it to flow in a direction transverse to the desired direction of flow thereby causing similar losses.
Furthermore, it has frequently been found that it is desirable to have relatively wide spacing between the adjacent folds of the heat exchanger sheet. The more desirable heat conductive materials from which the core is constructed, however, are limited, due to the desirability of having a very thin sheet material separating adjacent fluid flow passages and due to the characteristics of the material itself, in the depth to which these spacer dimples can be drawn.